Write the number you are asked to factor at the top of a piece of paper.
Determine two numbers that can be multiplied together to create the number you wrote down. Draw a small, vertical line under the first number slightly to the left. Write the first factor under this line. Draw a second vertical line under the first number slightly to the right. Write the second factor under this line.
Calculate two numbers that can be multiplied to create the first factor of the larger number. Draw two vertical lines under the first factor, and write the two numbers. Determine two numbers that can be multiplied to create the second factor. Draw two lines under the second factor, and write these two numbers.
Continue these steps of breaking down numbers until you reach a prime number, or a number that is only divisible by itself and 1. Your factor tree is finished when you reach all prime numbers. The resultant prime numbers are the factors of the number you started with.